1. Field of the Invention
This invention relates to the exposure of a charged photoconductor member to create latent image potential test patches useful in controlling process parameters in electrophotographic machines.
2. Description of the Prior Art
In electrophotographic machines such as printers and copiers, control of image density is required to produce copied images having constant and predeterminable image densities. Process parameters which determine image density include charger energization, exposure lamp illumination, development voltage bias, toner concentration in the developer mixture, and image transfer potential.
There are process parameter control methods known in the prior art wherein at least two test patches of different latent image potentials are formed on respective portions of a non-image area of the photoconductor member. The patches are developed, and the resulting toner densities of the patches are measured. The measured densities are used to adjust the image density parameters.
Generally, the test patches are created by exposing the photoconductor member to optical marks (one white and one black) on the frame portion of a platen on which original documents are laid. This provides two test patches of different densities which are useful to detect changes in sensitometric curves at widely spaced positions along the curve. Since one of the parameters which affects the patch density is exposure intensity, intentional changes in such intensity in response to operator-initiated changes in exposure settings to lighten or darken the copies will result in changes in patch densities. If the patch density measurement is used, without compensation for changes in exposure settings, the machine will attempt to adjust itself to return the measured patch densities to normal values. On the other hand, unintentional changes in exposure intensity needs to be detected and compensated for.